Best of the ISP-Lists

The Secret Art of Oversubsciption

With prices set against independant ISPs, it's an open secret that every line must have more subscribers than it could serve if they all logged on at the same time. Members of the ISP-Bandwidth list discuss the consequences.

[June 25, 2002]

On the ISP-Bandwidth list in June, PW asked,

"How do you define oversubscribing? Say you have 100 DSL customers with 7 Mbit down and 1 Mbit up, you have a 100 Mbit uplink, and your actual usage is never above 50 Mbit. Would you call that oversubscribing, or would you only call it oversubscribing if your circuit routinely hits saturation?"

A number of respondents contended that it really depends on how well it's being administered:

[HL explained] "My rule of thumb is that if latency, jitter, or throughput suffers noticeably adversely, it's oversubscribed."

[JH offered] "Sometimes, in order to properly utilize bandwidth on a circuit, one must 'channelize' that circuit to multiple customers. It is only a problem when one does not monitor or care about utilization, and tries to force more users on a circuit to compensate for a poor business model."

[HL laughed] "'Oversubscription' has gotten about as meaningless as 'Tier 1.'"

Others discussed the ethical side of the issue:

[JO contended] "Oversubscription allows for one carrier to charge more for the same services as another carrier."

[DK countered] "Or rather, oversubscription allows some carriers to collect more revenues than other carriers, by properly utilizing their pipes. Still, I'd be surprised if anyone doesn't oversubscribe at all—it would be very expensive for them not to."

Still others suggested that it's an expected way to run things:

[DK noted] "Oversubscribing is a normal and accepted part of the business. However, you can go too far with it by letting your circuits become saturated. Basically, assuming all of your customers are using 7 Mbit down at the same time, you can put about 14 customers on a 100 Mbit circuit. However, you know that your customers will rarely all use 7 Mbit at the same time. Therefore, you can get away with putting more customers on the circuit. During peak periods, they'll experience some lag, but most of the time, they should have a good experience."

[FM observed] "Oversubscription is as old as telecom. It probably was born when someone discovered that most telephones are idle most of the time. All carriers have to do it: it's the only way to make money. It is a science as much as an art. Statistical models are only so accurate, and experience is needed to oversubscribe in such a way that the end users don't perceive it. The whole field of traffic engineering could easily be renamed 'how to oversubscribe.'"

— End